In 3GPP (3rd Generation Partnership Project) which is a standardization project of a mobile communication system, a specification of a Device to Device (D2D) proximity service is proceeding as a new function after Release 12 (see Non Patent Document 1).
The D2D proximity service (D2D ProSe) is a service that enables direct device to device communication in a synchronous cluster made up of a plurality of synchronized user terminals. The D2D proximity service includes a D2D discovery procedure (Discovery) that discovers a proximity terminal and D2D communication (Communication) that is the direct device to device communication.
In addition, the discovery procedure in which a user terminal present in a certain cell (“serving cell”) discovers a proximity terminal present in another cell (adjacent cell) is referred to as an inter-cell discovery procedure (Inter-Cell Discovery). In addition, the D2D communication in which the user terminal present in the serving cell communicates with the proximity terminal present in the adjacent cell is referred to as an inter-cell D2D communication (Inter Cell Communication).